This invention relates to a system for electron beam exposure where the size of the electron beam is varied in accordance with the accuracy required for each of the patterns to be exposed.
An electron beam is used to expose an extremely fine pattern on the substrate of integrated circuits (IC). Several proposals for electron beam exposure systems have been tried, for example, systems using a Gaussian type beam (an electron beam where the cross-section of the beam current density has a Gaussian distribution), a shaped type beam (a rectangular electron beam) and a variable type beam.
Recently, the process for electron beam exposure has shifted to using an electron beam exposure system having a constant beam current density and a variable beam size or shape. A system of this type can provide a sharper pattern edge and constant irradiation by the beam on all portions of the pattern.
Processing speed, a very important factor in actual manufacturing assembly line use, is determined by the time necessary for the beam to irradiate the entire substrate. In order to reduce the processing time it is possible to increase the beam current density and the beam size, but both are limited by blurring or spreading of the beam which causes pattern edge variations. Thus, it is desirable to use the maximum beam size allowable for the required accuracy. The accuracy requirement of each pattern on a IC die differs for each pattern, so it is desirable to change the beam size from pattern to pattern. In the art, the word "pattern" sometimes refers to a figure formed on a die or chip of an IC or to a component of the figures (generally, rectangles) used to create shapes or figures of the IC circuit. In general, in the electron beam technology art the word "pattern" takes on the latter meaning. In the following disclosure, therefore, pattern is defined as a component used to create the figures, that is, the figures on the IC circuit are divided into rectangular patterns, and each of the patterns is individually exposed by an electron beam. There are large numbers of patterns, including very small patterns that can be exposed by only one shot (usually one pulse or on-off sequence of an electron beam is called a shot) exposure, and large patterns which need a large number of exposures (shots) in a series of steps to expose the entire area of the pattern.
The phrases "step and expose" or "step and shot" refer to a process used to expose a pattern larger than the beam size involving an exposure, shifting the beam by a pitch equal to the beam size and repeating the exposure, etc., until the entire pattern is exposed. The above phrases should be distinguished from the phrase "step and repeat", which is a process of exposing all the patterns on a die, stepping to the next die and exposing the same patterns on the next die, repeating the process on the same slice of IC.
As the scale of a large scale integrated device LSI gets large, the number of patterns in a die to be exposed is increased to a few million, and the number of shots necessary to expose the patterns thereon is increased to more than ten million. Thus, an improvement in the process speed or the throughput of the electron beam exposure system process is urgently needed.
In the prior art, beam size has been very difficult to control and, as a consequence, electron beam exposure has been performed using a constant beam size determined by the finest (or smallest) pattern on the die, so that it takes a long time to expose all the patterns on a slice.
In a step and expose method, the pattern size, beam size and pitch are given, and the beam is controlled by a computer in which the programming for the millions of patterns is a problem. When a pattern size and its position are given, the computer divides the pattern into rectangles equal to the beam size, and exposure of the pattern is performed by repeating the step and expose process until all the rectangles are exposed. Even though the calculation time for each pattern is short, the computer must perform millions of calculations in order to expose all of the patterns on a slice, the total time for the calculations for millions of patterns is very long and thus, the electron beam exposure process is becoming a bottle neck for LSI (large scale integrated circuit) manufacturing. The present invention intends to relieve this bottle neck.